1. Field of the Invention
The present invention relates generally to the field of electronic testing. More particularly, the present invention relates to devices and methods for mapping signal paths in electronic systems
2. Related Art
Electronic systems are ubiquitous. An essential component of these systems is their internal signal paths, most typically provided by wired interconnects. Failures in the wiring frequently result in failure of the system. For example, aging wiring in buildings, aircraft and transportation systems, consumer products, industrial machinery, etc. is among the most significant potential causes of catastrophic failure and maintenance cost in these structures. High profile airline crashes attributed to aging wiring have brought the need for improved wire testing systems to the forefront of industry attention.
Efforts to develop techniques for the characterization and fault detection of electronic signal paths have been underway for years, with many successes. For example, techniques such as time domain reflectometry (TDR), frequency domain reflectometry (FDR), and sequence time domain reflectometry (STDR) can be used to determine where a short or break in a wire has occurred. More recently, improvements such as spread spectrum time domain reflectometry (SSTDR) and noise domain reflectometry (NDR) have been developed to allow testing of a wire while operational signals are present.
A test instrument using these reflectometry techniques generally injects a reflectometry test signal into the wire to be tested. As the test signal propagates from the test instrument, impedance mismatches in the wire generate reflections that propagate back to the test instrument. Impedance mismatches can be caused by a variety of things, including for example, breaks in the wire, short circuits, branches, and wire gauge changes. These results are then measured either directly or indirectly, providing a reflectometry response of the wire under test. For example, in a TDR, the test signal is a fast rise time pulse, and the reflections of the pulse are observed on a display such as an oscilloscope. For a FDR, the test signal is a sine wave, and the frequency of the sine wave is swept or stepped in frequency to permit measuring the phase delay and associating this with corresponding mismatches. NDR operates slightly differently, as no test signal is injected. In NDR, an existing signal present on the wire is used as the test signal, and the reflections of the existing signal observed.
Interpreting the results obtained with a reflectometry instrument for anything other than simple wires typically requires great expertise, as the reflectometry response can be very complex. For example, mismatches can generate reflections to both a forward traveling signal (e.g., the test signal injected by the test instrument) and a reverse traveling signal (e.g., a reflection generated by a mismatch further down the line). Mismatches also affect the signal passing past the mismatch. For example, a pair of mismatches can result in an infinite, although decaying, train of reflections as a portion of the test signal bounces back and forth. In general, the resulting reflected and re-reflected signals within a network superimpose on each other to create a complex overall response.
For single wires, significant progress has been made in allowing automated interpretation of the reflectometry response, allowing useful information, for example, wire length, to be determined by the instrument and displayed to a user in an easily usable format. For example, distance to a mismatch can be determined by observing the delay between the injected test signal and the reflected signal.
Many electronic systems, however, use wiring that is interconnected into a branched network. Testing of networks of wires has proven challenging. This is because branches in the network add a further level of complexity to the reflectometry response. Junctions of wires create an impedance mismatch that can be difficult to distinguish from other types of impedance mismatch. Reflections from different branches of a network can be re-reflected by other branches, and overlap in time. The superposition of multiple reflections from different branches and branch ends can cancel each other out, reinforce, or otherwise combine so as to create erroneous distance measurements when simple analysis algorithms are used. The number of reflections also tends to grow exponentially with the number of branches in the network; hence, complex network topologies produce extremely complex reflectometry responses. As a result, even if a distance measurement to a fault can be obtained, the result may be ambiguous, since which branch the fault is located on is not provided. Analyzing networks of unknown topology can therefore be particularly difficult.